Chemical-mechanical polishing (CMP) is performed in the processing of semiconductor wafers and/or chips on commercially available polishers. The CMP polisher can have a circular rotating polish pad and rotating carrier for holding the wafer or, as with the newest tools entering the market, may be designed with an orbital or linear motion of the pad and carrier. In general practice, a slurry is supplied to the polish pad to initiate the polishing action. However, here again, newest tooling may make use of what is referred to as Fixed Abrasive pads, whereby the abrasive is embedded within the polish pad and is activated by DI water or some other chemical as may be desired for the specific polish process.
Ideally, a CMP polisher delivers a globally uniform, as well as locally planarized wafer. However, global uniformity on a wafer-to-wafer basis is difficult to achieve. Hard pads are used on a polishing table or platen for their ability to provide optimum planarity. However, these pads require a softer pad under layer to generate an acceptable level of uniformity. The application of wafer backside air is also a standard practice in an attempt to provide a localized area of force to the backside of the wafer in those radii where the polish may be lower due to wafer bow, collapse of the backing film, degradation or collapse of the polish pads, or poor slurry distribution.
Recently, a phenomenon known as "edge bead" has detracted from acceptable yields. The edge bead is a ring of thicker oxide at a radius of 96 mm with a 100 mm wafer. A secondary thickness variation at 80-90 mm has also been observed. The location of these thickness variations may also shift across the wafer unexpectedly for reasons not fully understood. This results in nonusable chips at the wafer perimeter or a variation in chip performance regionally across the wafer. Also, the wafer film to be polished may have a varying consistency from doping, thickness or the like, across the surface of the wafer. This creates varying, uncontrollable polish rates across the wafer. Neither of the problems described above can be compensated for with the tooling currently available.
Various mechanical methods have been attempted to alter the final thickness profile of a polished wafer. One method uses fixed curvatures or shapings of the carrier face. These are directed to control only a centered edge thickness variation by bowing the carrier face at the center to supply a greater force at the wafer center. This provides an increased rate of polish center to edge.
Another known method applies shims to the carrier face behind the wafer backing film. This enables a wider range of diameters and widths to be rotated on and off a flat carrier as needed. However, the milling of a carrier face to a shape requires a number of carriers to provide a range of results. This requires substantial time to change from one shaped carrier to another as the need arises.
The present invention is directed to overcoming one or more of the problems discussed above, in a novel and simple manner.